When I was 6, we had a first choice video in town. Whenever we when to it I would always request The Flight of Dragons. I remember my older brother not liking the movie. I now own it on VHS (yes, my family is that ancient). It is a very good movie for 6 year old's. Why did I say 7 and up you ask? It is because some of the scenes are questionable. As in a scene where Two of the dragons get drunk, and other scenes where monsters, and dragons fight. Other than that the movie is wonderful for 6 year olds.
The animation was a little scary when I first watched it as a kid. The graphics are simple but very detailed like a Arthur Rickman illustration. So when there is a closeup of characters whether good guy or bad guy, it makes for a sometimes chilling and frightening watch for kids. Overall though after I watched it as an adult, the story has some positive messages for kids about picking science over magic, using magic as a motivator for our hopes, dreams, and work, and whether the main villian is any indication of the society we live in and people we have become.